Light emitting diodes (LEDs) are emerging as light sources for many applications.
By application of wavelength converting materials, such a fluorescent and/or luminescent materials, in the path of light, the emitted wavelength can be adapted to many specific wavelengths. Blue and/or UV-light emitting LEDs are especially suitable as the light source in such light emitting diodes (hereinafter referred to wavelength converted light emitting diodes) due to that the wavelength converting materials typically absorbs at least part of the light emitted by the diode and emits light having a higher wavelength (a red-shift).
The wavelength converting material may be adapted to absorb essentially all light emitted by the LED (often referred to as pump-light) so that only light of the converted wavelength is output from the device. In other wavelength converting LEDs, the wavelength converting material is adapted to only absorb and convert a portion of the pump-light, so that the total output is a mixture of converted and unconverted light. For example, a partial conversion of blue light into yellow light results in a whitish total output.
One example of such a wavelength converted LED is described in WO 2007/085977 A1, to Koninklijke Philips Electronics N.V., where a plate of a ceramic conversion element is arranged on a LED-chip for conversion of part of the light emitted by the LED-chip.
However, the degree of conversion of the pump light is a factor of the distance of the light path through the wavelength converting material. Especially at the edges of the wavelength converting material, this may cause problems. At the edges, pump light have an opportunity to exit the device only after a short passage through the wavelength converting material, leading to a lower degree of conversion at these edges. This can often be seen as a ring of unconverted light surrounding the converted light.
Some wavelength converting diodes rely on self-supporting, such as ceramic, wavelength converting bodies that are bonded onto the light emitting surface of the LED-chip by means of a bonding material. In such diodes, the bonding material forms a spacing between the wavelength converting body and the diode, and by nature, the glue must be transmissive to the pump light emitted by the LED-chip. In such devices, some pump light is prone to exit the device via the lateral surfaces of the bonding material, once again resulting in a ring of unconverted light surrounding the converted light.
Especially in devices where full conversion of the light is wanted, this ring-effect is much unwanted as it calls for absorptive filters to remove output unconverted light.